Gas discharge vessel



Dec. 5, 1961 KARL-HEINZ RUMPF ETAL 3,012,166

GAS DISCHARGE VESSEL 2 Sheets-Sheet 1 Filed March 26, 1956 ATTO R N EYSDec. 5, 19%

Filed March 26, 1956 Tuclj- Uy I KARLHEINZ RUMPF ETA].

GAS DISCHARGE VESSEL 2 Sheets-Sheet 2 Impulse Gene/afar ATTO R N EYSnited States 3,012,166 Patented Dec. 5, 1961 doc 3,012,166 GAS DISCHARGEVESSEL Karl-Heinz Rnrnpf, Fredersdorf, near Berlin, and Roland Gessner,Berlin, Germany, assignors to VEB Werk fiir Fernmeldewesen,Beriin-Gberschonweide, Germany Filed Mar. 26, 1956, Ser. No. 574,359 15Claims. (Cl. 313-493) The invention relates to a gas discharge vessel,and more particularly to a gas discharge vessel or tube having a coldcathode and which is particularly suitable as switching means in anelectronic communication exchange.

It is known that present gaseous discharge electron tubes can not becontrolled similarly to electron vacuum tubes by means of a gridelectrode. Generally, the discharge in a cold cathode gaseous tube isinitiated by raising the voltage between the anode and the cathode to avalue greater than the ignition voltage and the discharge can beextinguished only by lowering the anode cathode voltage to a value lessthan the glow voltage of the tube. In thyratron tubes, a discharge maybe initiated from the thermionic cathode by reducing the control gridbias, but here again, the discharge can be extinguished only by loweringthe anode voltage below the value of the glow voltage.

An object of the present invention is to provide a cold cathode gaseousdischarge tube in which the grid is formed and located with respect tothe anode and the cathode so that the discharge in the tube can beignited and extinguished merely by applying a small control voltage tothe grid, which is small relative to the anode voltage, such control ofthe gaseous discharge being efiected without controlling the anodevoltage.

Another object of the invention is to provide a cold cathode gaseousdischarge tube alternating current amplifier in which the dischargecurrent is responsive to a small alternating voltage applied to thecontrol grid of the gaseous discharge tube.

Further objects and advantages of the present invention will be apparentfrom the following detailed descrip tion thereof in connection with theaccompanying drawings showing, by way of example, some embodiments ofthe present invention. In the drawings FIG. 1 is a perspective view of afirst embodiment of the invention, the front part of the device beingremoved, the remaining parts being cut in the plane CD of FIG. 2,

FIG. 2 is a horizontal section taken along the line A-B shown in FIG. 1,

FIG. 3 is a perspective view similar to FIG. 1 of a second embodiment ofthe invention, the cut shown in FIG. 1 being made along the line G.-Hshown in FIG. 4, I

FIG. 4 is a horizontal section of the device shown in FIG. 3 of the lineEF of FIG. 3,

FIG. 5 is a diagrammatic illustration of a circuit having a tubeaccording to the invention,

FIG. 6 is a diagram for the explanation of shown in FIG. 5,

FIG. 7 is a circuit diagram illustrating the connection of anotherembodiment of the present invention, and

FIG. 8 is a wiring diagram illustrating another application of a tubeaccording to the invention.

Referring now to the drawings and first to FIGS. 1 and 2, a dischargevessel or tube of which only the base 1 is shown is provided with a coldcathode 4 shaped as a cylinder and connected to the base 1 by arelatively short wire 10 and'a relatively long wire 11 passing throughthe base 1, and serving as a potential lead to the cathode. The cathode4 surrounds, as shown, the other electrodes the device to be mentionedpresently, and is arranged between two circular mica sheets 2 and 8joined with each other by rods 7 which are connected, respectively, withwires 10 and 11.

An auxiliary anode 3 shown in FIG. 1 as a ring-shaped wire is arrangedon two supports 12 and 13 connected, respectively, with wires 15 and 16which are connected after passing through the base 1 to form a singleconnection 17 for applying a positive potential to the auxiliary anode3. Instead of being shaped as a ring .the auxiliary anode might also beshaped as a cylinder or, as shown in FIGS. 3 and 4, two rods the ends ofwhich are held by the mica sheets 2 and 8.

Furthermore, a grid-like electrode 5 is supported by two rods 18 and 19supported in turn by the mica sheets 2 and 8. The supporting rod 18 isconnected with a wire 20 passing through the base 1 downwards.Furthermore, a main anode 6 shaped as a rod arranged along the axis ofthe cathode 4 and the grid-like electrode 5 is supported by the micasheets 2 and 8 and connected with a wire 21 passing through the base 1.The whole is enclosed by a vessel (not shown) forming an envelope ortube which is flied with a gas or a vapor.

Referring now to FIGS. 3 and 4, the design of the gas discharge tube isessentially the same as that of the tube shown in FIGS. 1 and 2, andtherefore only the differences shall be described more in detailhereinafter. The first difference resides in that the cathode 4' isshaped as a body having two opposite plane walls 22 and 23 connected bypart cylindrical walls 24 and 25, furthermore, a second grid-likeelectrode 9 is arranged coaxially with and inside the grid-likeelectrode 5. Furthermore, the auxiliary anode is formed by two rods 3'and 3" extending parallel to the main anode 6 in the space between thecylindrical surfaces 24 and 25 and the grid-like electrode 5. The tworods 3 and 3" extend through the mica sheet 2 downwards into the base 1and are connected by the connections 15 and 16 with the singleconnection 17.

The operation of the gas filled tubes shown in FIG. 1 to 4 is asfollows:

The anode 6 is maintained by the DC. source, such as a battery 23 undera direct potential relative to the cathode 4 or 4'. The grid-likeelectrodes 5 or 9 are given a direct voltage, an alternating voltage oran impulse voltage termed hereinafter a control voltage. If this controlvoltage or voltages, the source of which is not shown in FIGS. 1 and 3,are given a value imparting to the electrons together with thepenetrating potential field of the anode an energy which is suflicientto enable electrons to pass through the grid and form a plasma, thedischarge vessel will be ignited. When the control voltage is reduced tothe initial value the tube will be extinguished since the grid-likeelectrodes form isles preventing the flow of electrons in the grid anodespace so that the onization is not maintained. Since no new electronsare formed at the cold cathode after finishing the discharge because apotential field sufiiciently strong for overcoming the work functiondoes not exist at cathode 4, 4. The plasma on the grid-like electrodeshaving a control potential lower than that of the anode becomesexhausted so that the tube is extinguished without decreasing the anodevoltage below the operating voltage of the tube. Thus, if the tube isused in a relay, the means for extinguishing the tube are considerablyreduced.

If it is intended to reduce the magnitude of the change of the controlvoltage, or expressed in other terms, if the ability of the tube torespond has to be increased, the same may be accomplished by means of apreionization in the cathode-grid space. In order to accomplish this,the auxiliary anode 3 in FIGS. 1 and 2 or the auxiliary anodes 3 and 3"in FIGS. 3 and 4 are arranged near the cathode and are given a slightlypositive potential with respect to the cathode 4 or 4' as indicated byconnecting the conductor 17 with a cell C near the negative terminal ofthe battery B. ,In consequence thereof the auxiliary anodes 3, 3 3"maintain an uninterrupted auxiliary discharge forming a plasma havingthe effect of a permanent source supplying electrons or ions which thusis equivalent to the emitting cathode 4 or 4', Thus, the operation ofsuch a tube at the beginning and end of a discharge is comparable tothat of a tube having a hot cathode. In other words, the grid-likeelectrode or electrodes 5 and 9, which are imparted a potential belowthe operating voltage of the auxiliary discharge, prevent by theformation of isles the entry of electrons in the gridanode space of thetube, and therefore an ignition thereof. 'Only at a corresponding valueof the control voltage the potential field causing the formation ofisles becomes sufiiciently low, so that the kinetic energy of theelectrons sufiices for a piercing of the formation of isles so that thetube is ignited. The end of the discharge is effected by an increase ofthe potential field of the isle forming control electrons so that theelectrodes may no longer pierce the potential field, the plasma in thegridanode space being broken up so that the discharge is finished. For asatisfactory operation of these tubes the size of the openings of thegrid-like electrodes should be smaller than the length of the free pathof the ions and electrons.

The permanent auxiliary discharge may be tolerated since the sameinvolves only currents of an order of a few hundreds of micro-amps.Thus, the input of several hundreds of these tubes is not larger thanthat of a single tube having a hot cathode.

Tests made with the discharge vessel according to the invention haveproved the correctness of the facts pointed out hereinabove. If desired,more grid-like electrodes may be provided as shown hereinafter inconnection with FIG. 7, these electrodes forming blocking electrodes orinitiating electrodes which allow to realize almost all switchingpossibilities occurring in communication circuits or controllingtechnique such as relay circuits, coincidence circuits, anticoincidencecircuits, and circuits with a continuous grid control such as amplifiercircuits;

Referring now to FIG. 5, a wiring diagram of a gas filled tube 30 isshown having a cathode 32 connected to ground 34, and an anode 36connected through a resistor 38 with a positive direct potential. Thegrid 40 of the tube 30 is connected over a condenser 42 with a source Uof an alternating voltage. A source of a direct voltage U is connectedthrough a resistor 44 with a point 46 intermediate between the grid andthe condenser 42. Rectangular impulses are taken from the anodes 36 by aconnection 46 connected with an intermediate point 48 of the anode 36and the resistor 38.

The operation of this device is as follows:

The alternating voltage U supplied to the condenser 42 connected withthe grid-like electrode 40 is relatively high with respect to the directvoltage U supplied over the resistor 44 to the grid-like electrode 40.In consequence thereof the tube is caused to ignite and to extinguishaccording to the alternating voltage U fed to the grid-like electrode40. At the anode 36 rectangular voltage impulses are generated therebywhich are taken off the tube 30 by the connection 46. The ratio of theimpulses and the gaps separating the latter may be arbitrarily changedby adjusting the direct voltage U and it is even possible to suppressentirely the rectangular voltage impulses taken from the connection 46.

FIG. 6 is a diagram in which the grid voltage U is plotted on thehorizontal negative axis, whereas the anode current I is plotted on thevertical axis of the system of coordinate shown in FIG. 6. Analternating voltage U is superposed to the grid voltage U and the curveK shown in heavy lines shows the characteristic of the tube 30. 7 Itwill be seen that the characteristic K consists of a part L coincidingwith the horizontal axis, a part M vertical to the horizontal axis, anda slightly increasing part N. The part L extends over the negative halfwaves and half the positive half wave of the alternating voltage U andonly during the highest part of the positive half wave of thealternating voltage U the anode current shown by the characteristic Kincreases suddenly in the part M to the part N representing an impulse Ooccurring in the connection 46.

Also a saw-tooth voltage may be used for igniting and extinguishing thetube shown in FIG. 5. The width of the impulse generated in this case isalso a function of the bias applied to the grid-like electrode 40. Whenin such an arrangement an alternating voltage is again superposed to thegrid bias, said alternating voltage having a frequency being low againstthat of the saw-tooth voltage, rectangular impulses will be generated atthe output circuit of the tube, said impulses having a frequencycorresponding to that of the saw-tooth voltage and a width correspondingto instantaneous values of the amplitudes of the second alternatingvoltage. Thus, this phenomenon represents a time modulation.

if the impulses occurring at the output of the tube are differentiated,and then imparted with the variable flank thereof as an igniting andextinguishing impulse, to a further discharge vessel according to theinvention, impulses of a constant frequency, amplitude and width areencountered in the output circuit of the latter discharge vessel, saidimpulses having a phase position coinciding with the instantaneousvalues of the second alternating voltage. Thus this phenomenoncorresponds to a phaseimpulse modulation. The expenses for thesemodulations circuits are considerably lower than those for the knowncircuits for the aforementioned kinds of modulation.

Referring now to FIG. 7, a coincidence circuit is shown with subscribersloops in an electronic exchange designed as a coordinate exchange. Thesubscribers are allotted cables having three conductors a, b, c, or a,b, 0, respectively. The conductors a, b, and a, b, are speech conductorswhich may be connected with each other by the contacts t, t' of a relayT inserted into the connection 50 connecting the anode 52 of a gasfilled tube 54 with a battery and the grounded cold cathode 56 of thetube 54. I

The conductors c and c carry impulses characteristic of the respectivesubscribers and having a rectangular shape. The rectangular impulses areproduced in a centrally arranged impulse generator. The tube 5-4 hasthree grids 58, 60, and 62 arranged between the cold cathode 56 and theanode 52. The grid 58 is connected by a conductor 64 with the conductor0, the grid 60 is connected by'a conductor 66 with the conductor c. Thegrid 62 is connected by a conductor 68 with a conductor 70 carryinganother impulse sequence characterizing the I connection to be efiectedby the tube 54 as free or engaged, thus implementing acoincidence-anticoincidence efiect.

The operation ofthis device is as follows:

Under ordinary conditions the tube 54 does not carry any current sincethe grid-like electrodes 58, 60, and 62 thereof are not imparted anycontrol voltages from the conductors c, c or 70. However, when impulsescharacteristic for the first subscriber represented by the conductors a,b, c, are imparted to the conductor c by the central impulse generatorin cooperation with an impulse sequence imparted to the connection c" ofthe second subscriber by the central impulse generator (not shown), thetube- 54 becomes conductive so that the relay T' is energized andconnects with the contacts t, t thereof, the speech conductors a, b ofthe first subscriber with the speech conductors at, b of thesecondsubscriber. The saw-tooth voltage imparted to the conductor 70 andtransmitted to the third grid-like electrode 62 has the effect that theconnecting elements T, t, t associated with the tube 54 arecharacterized as free or engaged as the case may be. When theconversation between the two subscribers is finished the grid-likeelectrodes 58, 60, and 62 interrupt the flow of current through the tube54 and the relay T connected in the anode circuit thereof.

In a tube such as that shown in FIGS. 3 and 4 provided with twogrid-like electrodes 5 and 9, the occurrence of an impulse on one of theelectrodes will cause the tube to ignite, whereas the occurrence of theimpulse on the other of the grid-like electrodes will extinguish thetube. However, it should be noted that in the embodiments shown in FIGS.14, special forms of the electrodes of the tubes and specialarrangements thereof are shown, whereas, in the embodiments shown inFIGS. 5, 7, and 8, the arrangement of the cathode, the gridlikeelectrode or electrodes, and the anode is of a conventional kind.

Referring now to FIG. 8, an arrangement is shown in which the ignitedtube operates simultaneously as an amplifier. The gas filled tube 86includes a cathode 82 connected to ground, a grid-like electrode 84, andan anode 86. An anode resistor 88 connected to the anode 86 is connectedto a supply of a direct anode voltage. A conductor 90 is connected to apoint 92 between the anode 86 and the anode resistor 88. The grid-likeelectrode 84 is connected by a connection 94 with a condenser 96connected to a terminal 98 to which an alternating voltage U having arelatively small amplitude to be amplified is applied. The conductor 94is connected with a conductor 100 connected over a grid resistor 102 tothe movable contact 104 cooperating with stationary contacts 106 and 108which are connected, respectively, with a terminal 110 and a terminal112. To the terminal 112 a voltage U is applied eifecting an ignition ofthe gas-filled tube 80, whereas to the terminal 112 a direct voltage Uis applied which effects the extinction of the gas-filled tube 80. Inoperation, the alternating voltage U supplied to the terminal 98 isamplified by the tube 80 as long as the latter is ignited, so that inthe conductor 90 an amplified alternating voltage U is obtained.

In a tube having a cold cathode it is impossible to end the discharge byapplying a countervoltage to the control electrode. In order toaccomplish this, the anode voltage has to be lowered below the ignitionvoltage which may be efiected by applying an alternating voltage or astrong negative voltage impulse to the anode. However, this can only beaccomplished with considerable switching means which are avoided in thetubes and the connections according to the present invention.

It is seen from what has been said hereinbefore that the inventionprovides a gas filled discharge vessel or tube having a cold cathode,said discharge vessel being free of the defects of the discharge vesselsknown in the art, a particular advantage of the tubes according to thepresent invention being that the gas discharge may be finished while theanode voltage is applied. In general terms the present invention relatesto a gas discharge vessel or tube in which with a direct voltageconnected to the anode the gas discharge is started and terminated bychanging the values of a voltage or potential applied to a grid-likeelectrode.

The favorable efiects obtainable thereby have been described hereinabovein connection with the operation of the embodiments of the inventionshown in FIGS. 1-4.

Tests carried out with a discharge vessel according to the presentinvention have shown that the anode current thereof is suflicient forcommanding any control actions occurring in practice in remote signalingor control devices.

A preionization may be efiected, for instance, by means of ahigh-frequency field acting upon the gas filling the discharge tube.

With a plurality of grid-like electrodes the invention may be carriedout with a direct voltage applied to the anode by starting orterminating the gas discharge by changing at least one voltage orpotential applied to the grid-like electrodes. However, it is alsofeasible to start or extinguish of the gas discharge by changing thevalue of the voltage applied to one of the grid-like electrodes, and toprevent or start the discharge by a simultaneous change of the values ofthe voltages or potentials applied to a plurality of grid-likeelectrodes.

By this means it is accomplished that the discharge vessel according tothe present invention may be operated as a coincidence tube, ananticoincidence tube, or as a normal tube.

The present invention may be realized with a plurality, for instancethree grid-like electrodes, and a direct voltage applied to the anode,by starting or extinguishing the gas discharge at a constant value ofone of the control voltages, and changing the values of the othercontrol voltages applied to the grid-like electrodes, and to prevent orstart the gas discharge by changing the values of all control voltages.

By these means it is accomplished that the gas discharge vesselaccording to the invention acts in relation to the differentlycontrolled circuits, simultaneously as a coincidence tube, ananticoincidence tube, and a normal tube.

Tests carried out with tubes according to the invention have shown thatin the ignited state of the discharge vessel, according to theinvention, the operating voltage thereof, and thus the anode current maybe changed by means of one or more of the control voltages. During thedischarge the control voltage may be adjusted in a simple manner, forinstance, by means of a potentiometer or the like. If a direct voltageis used as a control voltage, as indicated in FIG. 8, and this directvoltage is superposed by an alternating voltage having a smallamplitude, this alternating voltage being applied to the terminal 98,the anode current follows the frequency and amplitude of the superposingalternating voltage applied to the terminal 98. The dependence of theanode current on the superposing alternating voltage has the advantagethat, for instance, an audio-frequency voltage apphed to the grid 84 ofthe ignited tube appears correspondingly amplified in the output circuitof the tube.

The discharge vessel according to the present invention may be used withadvantage as a coupling element in alternating voltage circuits, forinstance, audio-frequency circuits of electronic selector circuits, asshown hereinabove in connection with FIG. 7, such a use being impossiblewith the cold cathode tubes known in the art.

Also, by means of the discharge tube according to the invention,circuits and particularly control circuits in electronic selectorcircuits may be directly closed and opened, which was hitherto realized,with the full anode voltage applied, only by means of particularconnections generating extinguishing impulses. The use in electronicselecting circuits has great advantages consisting, apart from savingthe heating power and special devices for the withdrawal of heat, inthat when a direct voltage is applied as a control voltage, and a rtherdirect voltage as anode voltage, no humming is transferred to the longdistance line.

We have described hereinabove preferred embodiments of gas filleddischarge vessels or tubes and connections using the same. However, wewish it to be understood that many changes, alterations, orsubstitutions of equivalents may be made in the tubes and theconnections thereof described hereinabove without sacrificing any of theinherent advantages of the present invention which is defined by theappended claims.

We claim:

1. A gas glow discharge tube comprising an envelope enclosing a coldcathode, a grid electrode and an anode, grid electrode means locatedbetween the anode and the cathode and formed so as to be opaque toelectrons and ions in response to a given grid electrode biasing voltagerelative to the cathode, and means including said grid a e electrodemeans for initiating and extinguishing a gaseous glow discharge betweenthe cathode and the anode in response to biasing voltages on the gridelectrode relative to the cathode which are less than the low voltage ofthe tube and while the anode is at a positive voltage with respect tothe cathode which is in excess of the glow voltage.

2. A gas glow discharge tube comprising an envelope enclosing a coldcathode, a perforated grid electrode and an anode, said grid electrodebeing located between the cathode and the anode and formed so as toseparate the anode from the cathode, the perforations in the gridelectrode having a size smaller than the length of the mean free path ofthe ions and electrons, whereby said grid electrode is capable ofinitiating and extinguishing a gaseous glow discharge between the anodeand cathode in response to a control voltage which is smaller than theglow voltage of the tube.

3. A gas glow discharge tube comprising an envelope enclosing an anodein the form of a rod, a cylindrical grid electrode surrounding theanode, the openings in the grid electrode being smaller than the lengthof the mean free path of the electrons, a cylindrical cold cathodesurrounding the grid electrode and control means including said gridelectrode for extinguishing a gaseous glow discharge between the cathodeand the anode in response to a control voltage applied between the gridelectrode and cathode which is less than the glow voltage and while avoltage at least equal to the glow voltage is applied between the anodeand the cathode.

4. A gaseous glow discharge tube according to claim 3, wherein saidcontrol means includes a plurality of coaxial cylindrical gridelectrodes interposed between the cathode and the anode.

5. A gaseous glow discharge tube according to claim 3, including anauxiliary anode located between the cathode and the grid electrode formaintaining a continuous gaseous discharge between said cathode and saidauxiliary anode in response to a steady voltage connected therebetween.V V

6. A gaseous glow discharge tube according to claim 5, wherein saidauxiliary anode and said steady voltage are dimensioned to produce acurrent of the order of a few hundred microamperes.

7. A gaseous glow discharge device comprising a gas discharge tubehaving an envelope, an anode, voltage supply means for applying avoltage between the anode and the cathode which is in excess of theignition voltage of said tube, and means for initiating andextinguishing a gaseous glow discharge between the anode and the cathodewhile said voltage supply means is connected therebetween, said meansincluding a grid electrode located between said anode and cathode, theopenings in said grid electrode being of a size smaller than the lengthof the mean free path of the electrons, and means for applying a biasingvoltage between the grid electrode and the cathode which has a valueless than the voltage on said anode.

8. A device according to claim 7, including means for impressing analternating current signal between the control electrode and the cathodeand means connected to the anode for deriving an alternating voltageoutput therefrom. V

9. A deviceaccording to claim 7, including means for maintaining acontinuous gaseous discharge from said cathode, said last meanscomprising an auxiliary anode located between the cathode and the gridelectrode and a source of voltage connected between the cathode and theauxiliary anode.

10. A device according to claim 7, including a plurality of controlgrids and means for supplying control voltages to said grids.

11. A device according to claim 10, including three control grids andmeans for applying positive voltage pulses simultaneously to two of saidcontrol grids and means for applying a negative voltage pulse to theother control grid.

12. An alternating current amplifier comprising a gaseous discharge tubehaving an envelope enclosing an anode and a cathode, a direct voltagesource connected between the anode and the cathode, an alternatingvoltage source, control means connected to said alternating source forvarying the discharge current through said tube in accordance with thevoltage of said alternating voltage source, said control meanscomprising a perforated grid electrode connected to said alternatingvoltage source and interposed between the anode and the cathodeandsubstantially separating the anode from the cathode, the size of theperforations in said grid electrode being smaller than the mean freepath of the electrons and ions in said tube, and means for biasing thegrid electrode with respect to the cathode with a voltage which causesextinction of the gas discharge.

13. An alternating current amplifier according to claim 12, wherein saidbiasing means includes switching means for applying one biasingvoltageto said grid electrode for igniting a gaseous discharge in saidtube and for applying a second biasing voltage to said grid electrodefor extinguishing the gaseous discharge, said biasing voltages beingless than the anode to cathode voltage.

14. An arrangement for initiating and extinguishing an electronicplasma-type glow discharge in a gas filled electron tube, comprising agas filled tube having an envelope enclosing a cold cathode, an anodeand a control grid between the cathode and anode, said grid havingperforations of a size which is smaller than the mean free path or theelectrons, means for applying a positive bias potential to the gridrelative to the cathode of the magnitude of the plasma potentiahmeansfor applying a direct voltage between the anode and the cathode, meansfor applying a variable control voltage between the control grid and thecathode, said variable control voltage being of a sufiicient magnitudeto initiate the glow discharge in said tube when the plasma potential isexceeded and extinguish the glow discharge when the control grid voltagedrops below the plasma potential.

15. An arrangement according to claim 14, wherein said tube includes aplurality of control grids and wherein said means for initiating andextinguishing'the glow discharge applies a direct voltage to. one gridand said variable voltage to at least one other ofsaid plurality ofcontrol grids.

References Cited in the file of patent UNITED STATES PATENTS 1,145,735Ainsworth' July 6, 1915 1,789,626 Hendry Ian. 20, 1931 1,937,389 LangerNov. 28, 1933 2,077,288 Ardenne Apr. 13, 1937 2,292,382 Le Van Aug. 11,1942 2,443,205 Stutsman- June 15, 1948 2,468,417 Stutsman Apr. 26, 19492,616,986 Coleman Nov. 4, 1952

